The invention relates to footwear, more particularly to athletic shoes, wherein a cushioning sole is provided with a stability control device to enhance the stability of a wearer""s foot, particularly during lateral motion. The sole includes a sole member which is compressible and resilient to thereby cushion foot impact, with the sole member having a stability control device that enhances lateral stability.
Sole design for modem athletic footwear is generally characterized by a multi-layer construction comprised of an outsole, midsole, and insole. The midsole is typically composed of a soft, foam material to attenuate impact forces generated by contact of the footwear with the ground during athletic activities. Other prior art midsoles use fluid or gas-filled bladders of the type disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 of Marion F. Rudy. Although foam materials succeed in providing cushioning for the foot, foam materials also impart instability that increases in proportion to midsole thickness. For this reason, footwear design often involves a balance of cushioning and stability.
The typical motion of the foot during running proceeds as follows. First, the heel strikes the ground, followed by the ball of the foot. As the heel leaves the ground, the foot rolls forward so that the toes make contact, and finally the entire foot leaves the ground to begin another cycle. During the time that the foot is in contact with the ground, it typically rolls from the outside or lateral side to the inside or medial side, a process called pronation. That is, normally, the outside of the heel strikes first and the toes on the inside of the foot leave the ground last. While the foot is air borne and preparing for another cycle the opposite process, called supination, occurs. Pronation, the inward roll of the foot in contact with the ground, although normal, can be a potential source of foot and leg injury, particularly if it is excessive. The use of soft cushioning materials in the midsole of running shoes, while providing protection against impact forces, can encourage instability of the sub-talar joint of the ankle, thereby contributing to the tendency for over-pronation. This instability has been cited as a contributor to xe2x80x9crunners kneexe2x80x9d and other athletic injuries.
Various methods for resisting excessive pronation or instability of the sub-talar joint have been proposed and incorporated into prior art athletic shoes as xe2x80x9cstabilityxe2x80x9d devices. In general, these devices have been fashioned by modifying conventional shoe components, such as the heel counter, by modifying the midsole cushioning materials or adding a pronation control device to a midsole. Examples of these techniques are found in U.S. Pat. Nos. 4,288,929; 4,354,318; 4,255,877; 4,287,675; 4,364,188; 4,364,189; 4,297,797; 4,445,283; and 5,247,742.
In addition to the control of pronation, another type of foot motion in athletics also places xe2x80x9cstabilizationxe2x80x9d demands on athletic footwear. This type of motion is lateral, sideways or cutting movements which frequently happen in sports like basketball, volleyball, football, soccer and the like. An athlete in such athletics may be required to perform a variety of motions including movement to the side; quickly executed direction changes, stops, and starts; movement in a backwards direction; and jumping. While making such movements, footwear instability may lead to excessive inversion or eversion of the ankle joint, otherwise known as ankle sprain. For example, an athlete may be required to perform a rapid, lateral movement on a surface with friction characteristics that prevent sliding of the sole relative to the surface. Upon contact with the surface, the lateral portion of the foot impacts the interior of the footwear causing the lateral side of the midsole to compress substantially more than the medial side. The downward incline on the interior of the footwear caused by the differential compression, in conjunction with the momentum of the athlete""s body, creates a situation wherein the shoe rolls towards the lateral side, causing an ankle sprain. Similar situations which cause excessive inversion or eversion comprise one of the most common types of injury associated with athletic activities. A shoe with high lateral (side-to-side) stability will minimize the effects of differential compression by returning to a condition of equilibriumxe2x80x94tending to center the foot over the sole.
The preceding example particularly arises when footwear incorporates a midsole with cushioning qualities that sacrifice stability. In order to compensate for this lack of stability, designers often incorporate devices into the upper that increase stiffness. These devices attempt to provide a stable upper to compensate for an instable sole. Such devices take the form of rigid members, elastic materials, or straps that add to the overall weight of the footwear, make the article of footwear cumbersome, or restrict plantar flexion and dorsi flexion. For example, U.S. Pat. No. 4,989,350 to Bunch et al. discloses an article of footwear with sheet springs attached to the ankle portion, and U.S. Pat. No. 5,152,082 to Culpepper discloses an ankle support including a plurality of stiff projections extending along the heel and ankle. U.S. Pat. No. 5,896,683 to Foxen et al. discloses a support in the form of a plurality of finger-like elements attached to the upper which does not add significant weight to the shoe and allows plantar and dorsi flexion.
U.S. Pat. No. 5,343,639 to Kilgore et al., which is hereby incorporated by reference, discloses an athletic shoe wherein a portion of the foam midsole is replaced with foam columns placed between a rigid upper and lower plate. FIGS. 1 and 2 depict this prior art shoe. As seen in FIG. 1, four support elements are incorporated in the midsole. Shoe 10 includes conventional upper 12 attached in a conventional manner to sole 14. Sole 14 includes midsole 18, and conventional outsole layer 20 formed of a conventional wear-resistant material such as a carbon-black rubber compound. Midsole 18 includes footframe 23, cushioning and stability component 24, midfoot wedge 40 and cushioning layer 22 made of a conventional cushioning material such as ethyl vinyl acetate (E.V.A.) or conventional non-microcellular polyurethane (PU) foam extending substantially throughout at least the forefoot portion of shoe 10.
Midsole 18 includes cushioning and stability component 24 extending rearwardly approximately from the forefoot to a location adjacent the posterior portion of cushioning layer 22. Cushioning and stability component 24 includes shell or envelope 26 having upper and lower plates 28 and 30, defining therebetween an open area of the sole, and a plurality of compliant elastomeric support elements 32 disposed in the open area. In a preferred embodiment of this prior art shoe, elements 32 have the shape of hollow, cylindrical columns or columns containing a plurality of interior voids. Furthermore, the columns of the prior art have flat upper surfaces, the upper surfaces being parallel with the outsole.
Shell 26 may be made from nylon or other suitable materials such as BP8929-2 RITEFLEX(trademark), a polyester elastomer manufactured by Hoechst-Celanese of Chatham, N.J., or a combination of nylon having glass mixed therewith, for example, nylon with 13% glass. Other suitable materials would include materials having a moderate flexural modulus and exhibiting high resistance to flexural fatigue. Support elements 32 are made from a material comprising a microcellular polyurethane, for example, a microcellular polyurethane-elastomer based on a polyester-alcohol and naphthalene-1,5-diisocyanate (NDI), such as the elastomeric foam material manufactured and sold under the name ELASTOCELL(trademark) by BASF Corporation. Other suitable polyurethane materials such as a microcellular polyurethane-elastomer based on a polyester-alcohol and methylenediphenylene-4,4xe2x80x2-diisocyanate (MDI) and a microcellular polyurethane-elastomer based on a polyester-alcohol and bitolyene (TODI) may be used. These materials exhibit a substantially uniform cell structure and small cell size as compared to the non-microcellular polyurethanes which have been used in prior art midsoles.
According to the ""639 patent, utilization of microcellular polyurethanes has several advantages. For example, microcellular polyurethanes are more resilient than non-microcellular polyurethanes, thereby restoring more of the input energy imparted during impact. Furthermore, microcellular polyurethanes are more durable. This latter fact combined with the fact that the deflection of a foam column made from microcellular polyurethanes is more predictable than for non-microcellular polyurethanes allows the midsole to be constructed so as to selectively distribute and attenuate the impact load.
With reference to FIG. 2a, shell 26 includes upper and lower plates 28 and 30 which define an interior volume. Shell 26 serves to increase torsional rigidity about the anterior-posterior axis of the shoe. Additionally, shell 26 helps distribute the load between support elements 32, thereby controlling foot motion and providing stability. In FIG. 2a, upper and lower plates 28 and 30 are joined such that shell 26 has the shape of a generally closed oval envelope. This embodiment has the advantages of ensuring that all of the columns are loaded substantially axially during footstrike, and of providing a torsional restoring moment to upper plate 28 with respect to lower plate 30 when the foot is everted or inverted. Thus, stability is enhanced, making this embodiment particularly useful in running shoes. In addition, the closed envelope limits the load on the adhesives which secure support elements 32 to shell 26. Midfoot wedge 40 is disposed at the front of shell 26 and prevents total collapse of the shell structure at this region, which would cause a loss of midfoot support.
As depicted in FIGS. 2b and 2c, upper and lower plates 28 and 30 need not be joined and could take the form of unconnected upper and lower plates, or could be joined in only one portion, for example, the front or back.
Support elements 32 may have an overall hollow cylindrical shape and may have smooth exterior surfaces. Alternatively, the outer surface may include spaced grooves formed around the entire circumference on the exterior surface. Support elements 32 may be made from the elastomeric foam materials discussed above such as microcellular ELASTOCELL(trademark) or other microcellular elastomeric materials having the same properties.
As shown in FIGS. 2a-2c, four support elements 32 may be disposed between the upper and lower plates. Elements 32 are generally disposed in a rectangular configuration, with a pair of anterior lateral and medial elements and a pair of posterior lateral and medial elements. Elements 32 are secured to the upper and lower plates by detents 34 and a suitable adhesive such as a solvent based urethane adhesive.
The use of microcellular as opposed to non-microcellular polyurethane foam for the columns allows for the gradual increase in stiffness to be obtained without having the stiffness be too great or small at the location of the initial impact.
Accordingly, it can be seen that a midsole according to the prior art included a plurality of hollow elements constructed from a microcellular foam material such as ELASTOCELL(copyright) NDI improves over the prior art non-microcellular polyurethane foams by providing a lower stiffness at the location of the initial impact which corresponds to lower initial loads, and a smooth transition to a much higher stiffness corresponding to the maximum load which is achieved beneath the calcaneus, with the higher load distributed throughout the rear of the midsole. In addition, the desired stiffness is achieved in a manner which avoids bottoming-out throughout the ground support phase, without increasing the weight and initial stiffness of the midsole beyond a desired level.
As noted, the prior art disclosed that the outer surface of support elements 32 may be escalloped to include a plurality of spaced grooves extending around the entire circumference of support elements 32. The use of an escalloped outer surface provides the advantage that large vertical compressions are facilitated by the pre-wrinkled shape, that is, the columns tend to be deflected more vertically. If the columns are designed with straight walls rather than escalloped walls, the tendency of the column to buckle is greater.
The present invention is directed to enhancing the lateral stability of shoes which use a cushioning and stability component of the type disclosed in the ""639 patent.
The present invention relates to an article of footwear having an upper and a sole attached to the upper. The sole includes one or more support elements formed of a resilient, compressible material, and which are designed such that impact forces generated by movements of the wearer deflect the support elements in a manner producing a force directed to center the wearer""s foot above the sole.
Directed deflection of the support elements is achieved by using a support element with a canted upper surface. Unlike the support elements as disclosed in the ""639 patent that have a flat upper surface, the support elements of the present invention utilize an upper surface with a downward slope directed toward the interior of the footwear. In order achieve directed deflection, the support elements are arranged such that portions of the support elements on the exterior of the footwear have a greater elevation than portions on the interior of the footwear. When the support elements are located in the heel area, the heel of the wearer is positioned such that the periphery of the calcaneus is above portions of the support elements having lesser elevation. This arrangement ensures that the area of maximum stress is on the portion of the support element on the interior of the footwear, thereby causing the support elements to have a deflection bias in the inward direction.
Another aspect that adds to the directed deflection characteristics of the footwear are flexion indentations on the exterior of the support elements. In the ""659 patent, indentations around the entire exterior surface. By placing indentations in only a selected portion of the exterior surface, the column will bend in the direction that the indentations are placed relative to the support element. As such, flexion indentations placed on portions of the support elements facing the interior of the footwear create a second mode of deflection bias in the support elements that also facilitates bending toward the interior of the footwear.
In a preferred embodiment, the article of footwear contains two forms of support elements, cylindrical columns and an aft support. Both the columns and aft support include a canted upper surface. However, only the columns include flexion indentations. The convex shape of the aft support element, in conjunction with a high aspect ratio of width to thickness, creates an inward deflection bias similar to that of the columns.
The article of footwear of the present invention may also contain a rigid heel plate for receiving the heel of the wearer. The outer surface of the heel plate includes locations for attaching to the upper surface of the support elements. The heel plate surrounds the bottom, medial, lateral, and aft portions of the heel, thereby countering excess movement. In addition, the rigid heel plate uniformly transfers impact forces from the heel to each individual support element.
The columns can be formed integral with a base portion formed of the same resilient, compressible material as the columns. A base plate formed of generally rigid material may also underlie the base portion and the support elements.
Together, these features form a system wherein movement of the wearer, including lateral movement, generates a force that tends to center the foot above the sole of the footwear. While the primary use for the system is in the heel area, the system can be used in other portions of the shoe, such as in the forefoot. As noted, the downward cant and flexion indentations create a deflection bias in the support elements. When the footwear comes into contact with the ground, the wearer""s foot impacts the interior of the heel plate. The impact is then uniformly transferred through the rigid heel plate to the support elements. The deflection bias in the support elements tends to stabilize the heel plate and calcaneus above the sole. In a conventional article of footwear where the foam midsole has no deflection bias, the impact force will cause one area of the midsole to compress differentially from an opposite area. With the added momentum of the athlete""s body, inversion or eversion may result. In contrast, the deflection bias of the present invention causes the support members to deflect toward the interior of the footwear, thereby enhancing lateral stability. As such, this system provides an article of footwear with high lateral stability.